The present invention relates to a system for measuring electrophysiological phenomena generated in a human heart, and more particularly to providing novel approaches to not only inferably analyze electrophysiological phenomena generated in the heart by detecting potentials and magnetic fluxes generated therefrom but also visualize the analyzed results in consideration of easier interpretation and/or broader applications to cardiac diseases.
One conventional technique for the solution of the electrocardiogram and magnetocardiogram inverse problems is proposed by the inventors in a Japanese patent Laid-open Publication No. 8-289977. In this publication, a diagnostic system provides processing as shown by a flowchart in FIG. 1. The system has a computer in which a heart model is set therein with software, excitation conduction of the heart is efficiently simulated on the model using known methods, and an electrocardiogram or magnetocardiogram is obtained from the simulation results.
For using this approach, data including the shapes, positions, and directions of the ventricles, one or more early excitation positions and excitation time instants thereat, excitation conduction velocities in the ventricles, and conductance distributions in the ventricles are should be given as parameters.
In the above publication, the present inventors also propose a solution method of an inverse problem on the above propagated excitation simulation method. Specifically, some of the parameters (for example, early excitation positions and excitation time instants thereat) given to the ventricle model are corrected such that differences between measured electrocardiograms or magnetocardiograms and simulated electrocardiograms or magnetocardiograms with the above methods are lowered to values as small as possible. The possible lowest values are outputted as inferred results. The remaining parameters other than the parameters selected (for example, parameters for a distribution of propagated excitation velocities and a distribution of action potential amplitudes, except early excitation positions and excitation time instants thereat) are given as appropriate values, and those parameters are treated as fixed values in the inferring calculation.
Another conventional art is concerned with a method of inferring a distribution of excitation onset times on a ventricular surface from QRS complexes in multi-channels electrocardiograms detected the body surface, which is reported by the paper "H. Roozen et al., "Computing the activation sequence at the ventricular heart surface from body surface potentials", Medical & Biological Engineering & Computing, may 1987". In the paper, assumption is made that the conductance in a cardiac muscle is uniform and isotropic and its action potential amplitude is constant, and a volume integral equation (whose integral region is over the entire cardiac muscle) expressing the relationship between a distribution of excitation onset times in the cardiac muscle and QRS complexes guided channel by channel is converted into a boundary integral equation. By solving the equation, the distribution of the excitation onset times on the ventricular surface is analyzed from the measure results of QRS complexes in the electrocardiograms.
however, the forgoing conventional approaches still have various problems which should be overcome.
In the diagnostic system according to the first conventional approach, a magnetocardiogram and parameters to be inferred are in nonlinear relation. To infer the parameters requires calculation to be repeated tremendously many times and a simulation for propagated excitation to be performed every calculation. The simulation needs lots of calculation time. In consequence, a first problem is that time to perform one derivation of the simulation becomes extremely long.
A method of inferring intracardiac electrophysiological phenomena according to the first conventional approach is exemplified in a paper "T.IEE Japan, Vol. 116-A, No. 8, 1996, 698-704", where an early excitation region and its early excitation time are inferred as variables and distributions of both excitation onset times and action potential amplitudes are given as fixed values.
Owing to the fact that the paper treats a distribution of excitation onset times or action potential amplitudes as fixed values, a problem that inferred information does not fit to actual situations may be brought about. For example, if the inferring method according to the first conventional approach is actually applied to patients having myocardial infraction, a delay in propagated excitation will be caused by the infraction portion and an action potential amplitude will be decreased. Moreover, because an initial excitation portion and/or excitation time vary largely depending on individuals, it is difficult to treat as any fixed value an excitation conduction velocity distribution, action potential amplitude distribution, conductance distribution, initial excitation portion, and initial excitation time. In other words, these parameters should be treated as variables.
On one hand, if those parameters are all handled as variables, the number of parameters to be inferred outstandingly increases, the sizes of parameter spaces to be searched become huge, and it is extremely difficult to stably infer quantities. A second problem is that it is almost inadequate to diagnose abnormalities by applying in actual clinical fields the first conventional approach to patients having myocardial infraction or others.
On the other hand, in the case that the inferring method according to the first conventional approach is applied to patients who suffer from cardiac dysrhythmia and others, but not principal diseases such a myocardial infraction, it is considered that an excitation conduction velocity distribution, action potential amplitude distribution, and/or conductance distribution be given as fixed amounts, and only an early excitation region and early excitation time be inferred, as stated in the foregoing paper.
In this case, a steady analysis requires that propagated excitation velocities and action potential amplitudes in the cardiac muscle as well as a conductance distribution in the heart be given in detail. However, because information about those amounts is not known in detail at present, approximated information is compelled to be given. It is therefore difficult to simulate the propagated excitation in higher accuracy. A third problem is that inferred results are largely influenced by ambiguous amounts of propagated excitation velocities, action potential amplitudes, and conductance values, which have been pre-given, resulting in unstable inferred results.
It is actually frequent that patients who suffer from cardiac dysrhythmia have principal diseases, such as myocardial infraction or myocardial ischemia. A fourth problem is that, as long as an inferring system for intracardiac electrophysiological phenomena is used for patients who have only cardiac dysrhythmia, but do not have such principal diseases, applicable patients are extremely limited, thereby lowering versatility in application.
On one hand, the second conventional approach does not need the simulation for propagated excitation to be performed. Hence a problem that a huge amount of time needs for analysis is lightened. But the approach is based on the assumption that an action potential amplitude distribution and a conductance value distribution are uniform. This assumption is requisite when a volume integral equation adopted as a basic equation is converted into a boundary integral equation in this approach. It is therefore not allowed to apply the second conventional approach to patients who suffer from diseases, such as myocardial infraction. A fifth problem is that patients to which diagnosis according to the second conventional approach is applied are limited, lowering versatility, like the first conventional approach.
Moreover, there are some problems concerning a ventricular model and a chest conductance distribution model, which are caused in common from the first and second conventional approaches.
In the first and second conventional approaches, increasing inferred accuracy in an excitation onset time distribution and others needs to use a ventricular model fit to an individual ventricular shape in order to perform accurate analysis. However, the present situation is that the size and position of each patient's heart are measured by hand from his or her MRI image, and a typical ventricular model having a representative shape is made to enlarge or reduce according to the measured values.
it is difficult to automatically detect the heart size. Hence, completely extracting the heart shape from three-dimensional images acquired with MRI or X-CT in an automatic fashion is difficult. Producing a ventricular model fit to each of many patients as a routine work is not practical.
In magnetocardiogram inverse problems, in general, it is pointed out that a chest conductance distribution model fit to the chest shape and conductance distribution of each patient is required to be used in calculating magnetic fields. However, since the conventional approaches use the boundary element method or finite element method as means for taking account of patient's actual conductance distribution, it is required that tissue boundaries between the ventricles and the chest are configured with triangular meshes or the chest region is configured with tetrahedral meshes and other polyhedral meshes. The configuration requires to make the meshes fit to the tissue boundaries. Such fitting processing is difficult to be automatically performed. Thus it is actually difficult to produce the chest conductance distribution model routinely every day for each of many patients.
It is therefore concluded that an inferring method which considers the chest shape and the chest conductance distribution both of which is fit to each patient has not been practiced, lowering accuracy in inferring an excitation onset time distribution. This is a sixth problem posed by the conventional approaches.
On the other hand, for displaying analyzed results by the foregoing various approaches, typical display methods includes methods of displaying a single dipole, displaying a current source distribution, and displaying an excitation onset time distribution.
The method of displaying a single dipole concentrates on a conventional generally-used analysis that approximately assumes a single current dipole existing in the heart as a current or flux source, and on this assumption, infers the position, direction and magnitude of the current dipole. Thus, by this display method, those analyzed results are visualized with symbols such as arrows.
The cardiac muscle is in action, analyzed values of the position, direction and magnitude of a current dipole changes from time to time. Thus analysis during a certain interval produces a locus of the current dipole.
In the display of such locus, a topographic image acquired by clinical imaging modalities, such as MRI systems or X-CT scanners, is used. Generally, symbols such as arrows are placed on inferred positions on such topographic image. FIG. 2 shows one such example, which exemplifies that an inferred position of a single current dipole is in the cardiac septum and temporal loci (times 1 to 4) indicating changes in positions of the inferred current dipole are expressed by arrows.
However, applications that can be approximated by a single current dipole are limited; for example, only current sources activated in an internal corresponding to the early QRS, current sources corresponding to an early interval of ventricular extrasystole, or some other current sources can be approximated. In general, because current sources are spread in the heart, it is difficult to express such spread sources with a single current dipole. The display method relied on a single current dipole is, in fact, limited for use. Although this display is used for limited particular applications, such as inferring accessory conduction paths for WPW syndromes, or early excitation regions in ventricular extrasystole, it is almost difficult to apply this display to other diseases including myocardial infraction and cardiac dysrhythmia.
A method of displaying current source distributions is on trail. On the assumption that electroaction sources in the heart may be approximately expressed by current dipoles distributed therein, this display method is used to display in colors distribution data of inferred current dipoles. According to this approximate inferring, different sets of current dipole distribution data are obtained in a certain ventricular section for different analysis times. FIG. 3 illustrates density images indicating current dipole density distributions at times 1 to 3 in a certain interval. This display fairly improves difficulties encountered in the foregoing single dipole display.
However, the display method with current source distributions visualizes only a given section of the ventricles. When considering a situation that the ventricles should be observed from many directions thereof, different types of plural sections are required to be displayed. If a plurality of such images are produced and displayed, it is probably possible that temporal changes in current dipole distributions can be observed. If doing so, a number of images are required. Even if those images are produced and displayed, an operator is obliged to interpret a number of images displayed in turn. Of course, the interpretation involves bothersome operation such as scrolling images. Scrolling images is apt to lose information interpreted from the former images, having influence on accuracy in interpretation. Scrolling also increases operational work. Further, even when there are a plurality of monitors for interpretation, to observe the monitor screens separately needs plenty of interpretation work, lowering efficiency in interpretation.
This display method with current source distributions also poses a problem that resolution displayed are rather low, being inferior in providing effective diagnosis information.
A further method of displaying distributions of excitation onset times is proposed by, for example, the above-cited paper "T.IEE Japan, Vol. 116-A, No. 8, 1996, 698-704. According to this paper, ventricular excitation onset time distributions are analyzed based on measured magnetocardiogram data, and the distributions are visualized. Another approach for this kind of display is disclosed by "H. Roozen et at,: "Computing the activation sequence at the ventricular heart surface from body surface potentials", Medical & Biological Engineering & Computing, May, 1987". In this paper, excitation onset time distrubitons of the ventricular surface are inferred from QRS waveforms in electrocardiogram acquired from the body surface, and inferred results are displayed.
However, in these display methods, the analysis and display are restricted to the myocardial excitation conduction process. In case where lesions in excitation conduction, which includes bundle branch block, WPW syndrome, and ventricular extrasystole, with no action potential lowered, the displayed images are still useful. To the contrary, where diseases that involves large decreases in the action potential amplitude compared with troubles in excitation conduction, as seen from myocardial infraction or myocardial ishemia, it is difficult to distinguish diseases in the displayed images. Application for these display methods is also restricted.